Posts by Phil Tuma

Better Ways

"...will it ever fly in the datacenter, I doubt it but there are better ways."

I appreciate your sentiments. Believe me, I appreciate any and all skepticism concerning immersion cooling. After 15 years working as a liquid cooling Thermal Engineer, I can TRULY appreciate it.

I also appreciate the cost and complexity of all the liquid cooling technologies that HAVE succeeded (immersion among them). They include pumps, quick couplings, heat exchangers, manifolds, tubing, clamshells, hermetic IO connectors, and risk that will ultimately prove unacceptable in the context of commodity computational electronics, at least so long as there is a simpler, safer, cheaper and more efficient technology. I think this may be it. I’d like to know of a better way.

Limitations of Oil

Others ARE promoting mineral oil immersion for datacenter applications. Mess aside, the efficiency of those fluids is limiting, believe it or not. All dielectric coolants have miserable liquid phase heat transfer properties when compared with water. If you pump them as a liquid from a heat source (CPU) to a heat sink (water cooled heat exchanger) those shortcomings will manifest as a high source-to-sink temperature difference.

If, however, you have a dielectric fluid with the right volatility, using it in a 2-phase (evaporative) mode as we propose greatly increases heat transfer efficiency and decreases the resultant temperature difference. Also, as you noted, all devices are at the same temperature.

You are correct to stress the OTHER fluid characteristics as well. The fluids we propose are being used as Halon replacements to extinguish fires in manned environments. So far, they see quite viable for this type of application. Google “IEEE Tuma fluoroketone” for more info.

Spray Cooling vs Passive Boiling

The argument for spray evaporative cooling which was, as you noted, used in some Cray machines, is the elimination of the primary thermal interface material (TIM) between the silicon and the Integrated Heat Spreader (IHS). When you spray on the bare silicon, it is argued, this TIM is absent. Performance must be better right? It just isn’t true. The heat transfer coefficients achievable with spray evaporative cooling with volatile dielectric coolants are just too low.

Believe it or not, you’ll get better chip-to-fluid performance passively boiling off of a modern CPU package if the IHS is properly modified. This is true despite the addition of a TIM between the silicon and the IHS. Furthermore, the chip is protected by the IHS from any fluid-borne contaminants that might be left behind by fractional distillation of the fluid on the boiling surface.